In underground drilling, such as gas, oil or geothermal drilling, a bore is drilled through a formation deep in the earth. Such bores are formed by connecting a drill bit to sections of long pipe, referred to as a "drill pipe," so as to form an assembly commonly referred to as a "drill string" that extends from the surface to the bottom of the bore. The drill string is rotated, thereby causing the drill bit to advance into the earth, forming the bore. In order to lubricate the drill bit and flush cuttings from its path, a high pressure fluid, referred to as "drilling mud," is directed through an internal passage in the drill string and out through the drill bit. The drilling mud then flows to the surface through the annular passage formed between the drill string and the surface of the bore.
The distal end of a drill string, which includes the drill bit, is referred to as the "downhole assembly." In addition to the drill bit, the downhole assembly often includes specialized modules within the drill string that make up the electrical system for the drill string. Such modules may include sensing modules, a control module and a pulser module.
In some applications, the sensing modules provide the drill operator with information concerning the formation being drilled through using techniques commonly referred to as "measurement while drilling" (MWD) or "logging while drilling" (LWD). For example, resistivity sensors may be used to transmit, and then receive, high frequency wavelength signals (e.g., electromagnetic waves) that travel through the formation surrounding the sensor. The construction of one such device is shown in U.S. application Ser. No. 08/751,271, entitled "Apparatus for Joining Sections of Pressurized Conduit," hereby incorporated by reference in its entirety. By comparing the transmitted and received signals, information can be determined concerning the nature of the formation through which the signal traveled, such as whether it contains water or hydrocarbons. One such method for sensing and evaluating the characteristics of the formation is disclosed in U.S. Pat. No. 5,144,245 (Wisler), hereby incorporated by reference in its entirety. Other sensors are used in conjunction with magnetic resonance imaging (MRI) such as that disclosed in U.S. Pat. No. 5,280,243 (Miller), hereby incorporated by reference in its entirety. Still other sensors include gamma scintillators, which are used to determine the natural radioactivity of the formation, and nuclear detectors, which are used to determine the porosity and density of the formation.
In other applications, sensing modules provide information concerning the direction of the drilling and can be used, for example, to control the direction in which the drill bit advances in a steerable drill string. Such sensors may include a magnetometer to sense azimuth and an accelerometer to sense inclination.
Signals from the sensor modules are typically received and processed in the control module, which may incorporate specialized electronic components to digitize and store the sensor data. In addition, the control module may also direct the pulser modules to generate pulses within the flow of drilling fluid that contain information derived from the sensor signals. These pressure pulses are transmitted to the surface, where they are detected and decoded, thereby providing information to the drill operator.
As can be readily appreciated, such an electrical system will include many sophisticated electronic components, such as the sensors themselves, which in some cases include or are mounted on printed circuit boards, and associated components for storing and processing data in the control module, which may also include printed circuit boards. Unfortunately, many of these electronic components generate heat. For example, the components of a typical MWD system (i.e., a magnetometer, accelerometer, solenoid driver, microprocessor, power supply and gamma scintillator) may generate over 20 watts of heat. Moreover, even if the electronic component itself does not generate heat, the temperature of the formation itself may exceed the maximum temperature capability of the components.
Over-heating can result in failure or reduced life expectancy from such electronic components. For example, photomultiplier tubes, which are used in gamma scintillators and nuclear detectors for converting light energy from a scintillating crystal into electrical current, cannot operate above 175.degree. C. Consequently, cooling of the electronic components is important. Unfortunately, cooling is made difficult by the fact that the temperature of the formation surrounding deep wells, especially geothermal wells, is typically relatively high, and may exceed 200.degree. C.
Cooling methods used in applications associated with the monitoring and logging of existing wells, as distinguished from the drilling of new wells, typically require isolating the electronic components from the formation by incorporating them within a vacuum insulated dewar flask. One such approach is shown in U.S. Pat. No. 4,375,157 (Boesen), and includes thermoelectric coolers that are powered from the surface and that transfer heat from within the dewar tube to the well fluid by means of a vapor phase heat transfer pipe. Such approaches are not suitable for use in drill strings since dewar flasks are not sufficiently robust to withstand the shock, vibration and high pressures to which the down hole assembly of a drill string is subjected. Moreover, the size of such configurations makes them difficult to package into a down hole assembly.
Consequently, it would be desirable to provide a rugged yet reliable system for effectively cooling the electronic components that is suitable for use in a well. It would also be desirable to provide a cooling system that was capable of being used in a downhole assembly of a drill string.